Let us learn about Diversity in Modifications of Stems. After reading this article you will learn about: 1. Categories of Modification of Stems 2. Functions of Stems.
Categories of Modifications of Stems:
1. Underground Modifications of Stems:
Some stems develop underground in the soil for the purpose of perennation, vegetative propagation and storage of food.
These modifications of stems can be differentiated from the roots as having:
(a) Nodes and internodes,
(b) Scale leaves and
(c) Buds of axillary and terminal types.
Various types of underground stems are as follows:
(i) Rhizomes:
They are plagiotropic underground stems of the most varied kind. It is a prostrate, thickened stem, creeping horizontally under the surface of the soil. It is provided with distinct nodes and long or short internodes. It possesses a bud in the axil of scaly leaf and it ends in a terminal bud. The apex of a rhizome is always eventually transformed into an upright shoot, which becomes aerial.
As this apex grows through the soil it requires protection, which is afforded by smooth, hard and pointed scales which provide a boring point, as in Ammophila. The rhizome may be un-branched or sometimes the axillary buds grow out into short stout branches. It remains dormant underground and on the approach of favourable conditions the terminal bud grows into aerial shoot.
In certain cases the branches are separated off, each growing into a new independent plant. In the end of the season or after flowering the aerial parts die down every year and in the following year growth is resumed by one or more lateral buds and this process of growth continues year after year.
Normally the rhizomes are horizontal but sometimes they grow vertically upwards, e.g., in Alocasia. Examples of rhizomes are found in Carina, ginger, Curcuma, water lily, Elattaria, Amomum, lotus, ferns and many aroids.
(ii) Tubers:
The tubers are solid, thickened stems or branches serving for storage of food and also vegetative propagation. They are generally formed on rhizomes, on axillary branches or on main stems, either below or above ground. In the case of Cyperus esculentus (kaseru) the tubers are terminal on rhizome branches. Branch tubers are found in potato. The tuber bearing branches come from the lowest axils of the aerial stem.
These branches are weakly geotropic, and when formed above ground they bury themselves and swell at their apices to form the massive tubers. These tubers bear temporary scale leaves with buds in their axils called ‘eyes’. The tuber remains dormant for some time.
On its germination, orthotropic shoots are developed from the axiliary buds, which become new aerial stems. Even a single detached bud may grow into a new plant. The tubers of Helianthus tuberosus are developed like those of potato, but comparatively they may borne on short branches. They contain inulin instead of starch.
(iii) Bulbs:
The bulb is also a short specialized underground shoot or stem. In this case, the stem remains, comparatively small, and the food material is stored in the large, fleshy scales which invest and overlap the stem. However, the stem is no more than a thick disc or very flat cone but it has an apical bud on the upper side, and adventitious roots are formed in an annual plant from the marginal portion of the underside.
The bulb develops in the resting period or two to four months after flowering of the plant. The fleshy scales are set very close together and their bases of insertion partially or completely surround the stem. In scaly bulbs (e.g., Lilium), the fleshy scales composing the main bulk overlap at their margins. In tunicated bulbs (e.g., Allium, tulip, etc.) the outer leaves are large and completely en-sheath the inner portion of the bulb.
The bulbs are organs of perennation and of vegetative propagation. While still underground, the organs of the aerial shoot, including the parts of the flower, are already formed in their buds.
(iv) Corms:
The corm is a condensed form of rhizome and consists of a stout, solid, fleshy underground stem growing in the vertical direction. The swollen stem bears a number of loose’ more or less sheathing scale leaves. The size of the stem is due to the large amount of food material stored in it.
On top it bears an apical bud, from which summer shoots with leaves and flowers are produced, and it bears the annual adventitious roots, usually from the lower end. The examples are seen in Crocus and Gladiolus. In these cases the corm is considerably flattened and its length remains shorter than its diameter.
The axillary buds are occasionally found at the nodes on the corm, from which side shoots and new corms are given out. After flowering, the leaves of Crocus remain active for some time, and the material assimilated by them is stored at the base of the flowering stem, producing a new corm, on top of the old one, from an axillary bud. Other examples are seen in Colocasia, Amorphophallus, Colchicum, etc.
2. Sub-Aerial Modifications of Stems:
In some plants for the purpose of vegetative propagation some of the lower dormant buds of the stem grow out into lateral branches which according to their origin, nature and mode of propagation are known by different names, such as runner, stolon, offset and sucker. They may be sub-aerial or partly subterranean.
(i) Runner:
This type of slender, prostrate branch possesses distinct nodes and long internodes. It creeps on the ground and the roots are produced from the nodes. The runner develops as an axillary bud and creeps some distance away from the mother plant. The roots are given from the nodes and thus new plants develop, e.g., Oxalis, Cynodon, Fragana, Centella, etc.
(ii) Stolon:
It is a weak, slender, lateral branch that arises from the base of the shoot buried in the soil. It grows horizontally outwards, and where it touches the ground, the adventitious roots are given out. The branches grow out in different directions, and its terminal bud emerges out of the soil and develops into a new plant. The branch is subterranean. Examples are seen in Alocasia, Colocasia, jasmine, etc.
(iii) Offset:
The offset of a plant originates in the axil of a leaf as a short, thick, prostrate branch, and produces at the apex a tuft of leaves above and a cluster of roots below. It often breaks away from the mother plant and an independent daughter plant develops, e.g., Pistia, Eichornia (water hyacinth), etc.
(iv) Sucker:
Like the stolon the sucker also develops as a lateral branch from the underground part of the stem. It grows obliquely upwards and develops into a new plant. It is always much shorter than a stolon. The sucker develops roots at the base.
This is device for vegetative propagation, as seen in Chrysanthemum, mint, etc.
3. Aerial Modifications of Stems:
Vegetative and floral buds found on the stem develop into branches and flowers respectively. However very often this aerial stem undergoes extreme degree of modification in certain plants for definite purposes. Such metamorphosed organs may be stem tendril for climbing, thorn for protection, phylloclade for food manufacture, and bulbil for vegetative propagation.
(i) Stem-Tendril:
These are highly specialized climbing organs. They are slender often branched and may bear small scale leaves. The climbers by means of these tendrils attach themselves to neighbouring objects and climb them. The stem tendril is the modification of a stem is evident from the fact that it arises in the axil of a leaf or at the apex of a branch.
For example in passion flower (Passiflora) the axillary bud is modified into the tendril, whereas in vine (Vitis) it is the terminal bud which is modified into tendril.
(ii) Thorns:
True stem thorns arise from leaf-axils and often bear leaves or flowers as evidence of their branch nature. Examples are seen in Crataegus and Hippophae. The thorns arise in spring as axillary shoots with normal leaves and with an apical bud. The apex soon stops growth and hardens into a woody point, from which the undeveloped leaves fall away, leaving it naked.
In Ulex, the main shoots are thickly set with compound thorn branches, but here the leaves are also reduced to spines and the function of photosynthesis develops on the branches including the thorns, which are all green.
(iii) Phylloclade:
This is a green, flattened rounded stem, usually found in the plants of dry and arid habitats. This stem structure has taken on the general appearance and functions of a leaf. Usually the phylloclades represent lateral branches.
They possess distinct nodes and internodes and sometimes they bear modified leaves in the form of scales or spines. Examples are found in Opuntia, Cocoloba, Euphorbia, etc.
The phyllociade of one internode is known as cladode, e.g., in Asparagus, duckweed (Lenina).
In Ruscus it consists of two internodes. Here the cladodes are flat, leaf like and perform the function of photosynthesis like foliage leaves.
(iv) Bulbils:
They may be described as axillary buds, which become large and fleshy owing to the storage of food material in their leaves, it is a special multicellular body essentially meant for the reproduction of the plant. The bulbils differ from ordinary buds in the fact that they separate from the parent plant, fall to the ground and produce new plants, thus serving for reproduction.
The examples are found in Dioscorea, Globba, Agave, onion, etc.
Functions of the Stem:
1. Mechanical Support:
The main stem is usually thick, aerial and stout and keeps the plant in erect position. The main stem and the branches bear leaves and distribute them out on all sides so that all of them may get the maximum sunlight for photosynthesis. They also bear flowers and fruits.
2. Conduction:
The stem conducts water and minerals from the root to the leaf. It also transports the prepared food material from the leaf to the different parts of plant body, specifically the storage organs and the growing regions.
3. Food Storage:
In many cases, particularly in the underground modified stems, it serves as a storehouse of food material (e.g., in potato, ginger, taro, onion, Amorphophallus, etc.
4. Water Storage:
In many fleshy stems of cacti and euphorbias water is stored in sufficient quantity that is used by the plant in extreme dry conditions.
5. Photosynthesis:
The young green stems and the modified stems of many cacti and other fleshy plants manufacture food material in the presence of sun light with the help of chloroplasts present in them.
6. Protection:
Several modified stems into thorns protect the plants from grazing animals.
7. Perennation:
Several modified underground food laden stems also serve as perennating organs, e.g., potato, ginger, onion, taro, etc.
8. Vegetative Propagation:
Several aerial, sub-aerial and underground modifications of the stem help in vegetative propagation, runner of doob grass, sucker of mint and setts of sugarcane.